Determining thermodynamic variables of a fluid such as the thermal conductivity and the thermal capacity of a super-cooled liquid near the glass transition with the aid of the so-called 3ω method is described in N. O. Birge et al., “Specific Heat Spectroscopy: Origins, Status and Applications of the 3ω-Method”, Thermochimica Acta, 304/305, (1997), pp. 51 through 66, in which the theoretical fundamentals of the 3ω method and the procedure for determining the thermal conductivity and the thermal capacity of a fluid with the aid of this method are also explained in detail.
Moreover, a sensor element is described there in which planar heating elements having an electroconductive nickel coating are provided on a borosilicate glass substrate and are dipped into a fluid. A periodic heating current is applied to this sensor element via external electrical components, and the third harmonic wave of the electric voltage applied to the heating element is evaluated.
However, this sensor element is a laboratory design which is not suitable for routinely determining thermodynamic variables of a fluid. In particular, this sensor is not sufficiently compact to, for example, incorporate it in a motor vehicle and use it there for monitoring the condition of the oil.
In principle, when working with the 3ω method, a thermal diffusion wave is produced by transient heating, amplitude and penetration depth of this wave being small, so that thermal conductivity λ and thermal capacity cp may be determined in a substantially trouble-free manner, i.e. without significant temperature rise.
The object of the present invention is to provide a compact sensor element, able to be manufactured in series production, for continuously determining or monitoring the thermal capacity and/or the thermal conductivity of a fluid such as an oil in a motor vehicle.